This invention relates to a leakage current prevention circuit, the leakage current of which occurs when data writing is performed with respect to an EEPROM for example, EEPROM and a program voltage switching circuit.
FIGS. 3 and 4 are a plan view and a cross sectional view each of which illustrates part of a known MOS inverter of a semiconductor integrated circuit used in an EEPROM. In FIGS. 3 and 4, an NMOS 3 formed in a P-type semiconductor substrate (Psub) and a PMOS 2 (source 31 and drain 32) formed in an N well region 4 are connected to each other by a poly-silicon wiring 1 in order to provide a common gate. A conventional MOS pattern (for example the CMOS inverter formed in the Psub) was formed such that in order to reduce the cell size a PMOS gate and an NMOS gate were connected to each other by the poly-silicon wiring 1 in such a manner as to pass over a junction of the N well region 4 and the semiconductor substrate Psub.
However, in the MOS inverter according to the prior art, when a high voltage (Vpp) occurs at the time when data writing operation is performed with respect to the EEPROM, as illustrated in FIGS. 3 and 4 a leakage current flow path 33 from the PMOS to Psub was in some cases formed whereby leakage current flowed. That is, in case that the N well region 4 is at a potential of Vpp and the poly-silicon wiring is at a potential of 0, the semiconductor substrate surface of the N well region 4 right beneath the poly-silicon wiring 1 is inverted with the result that the leakage current path is inconveniently formed. As a result, leakage current flows along a path of source 31 of the PMOS 2, a channel of the PMOS 2 (P-type because of the on-state), the N well region right beneath the poly-silicon wiring (P-type because of the inverted state), and the semiconductor substrate Psub. Thus, the problem that such flow of leakage current adversely affects the writing characteristics exists.
The object of the present invention is to prevent, in order to solve the above problem, the occurrence of leakage current by cutting off part of the leakage current path.